1. Field of the Invention
The present invention relates to strain gage load cells, and more particularly to compression column load cells, both fixed axis and Rocker Pin load cells, with means for eliminating errors caused by off center loading of the compression column.
2. Description of the Related Art
A compression column load cell is the original design of a strain gage load cell. It comprises a compression column with a fixed vertical axis and a small radius spherical load button on top.
A Rocker Pin load cell is a special form of compression column load cell, in which the compression column is fitted with large radius spherical surfaces on both ends, so the load cell column can act as a self stabilizing rocker pin between parallel top and bottom loading surfaces. The Rocker Pin load cell was first described in U.S. Pat. No. 4,804,053. Since then, Rocker Pin load cells have been used extensively as load cells for truck scales and other high accuracy, heavy duty weighing applications.
All compression column load cells are basically columns of load cell quality material, such as steel or aluminum alloys, with strain gages bonded symmetrically around the compression columns near the midpoint of the columns, and fitted with housings or sealing bellows. The cross section of the compression column where the strain gages are bonded is usually square. Strain gages aligned with the longitudinal axis of a column are bonded to a first pair of opposing faces of the column to measure longitudinal strain, and strain gages aligned crosswise to the axis are bonded to the remaining pair of opposing faces of the column to measure transverse strain. The four strain gages are connected in a bridge circuit, which provides an electrical output signal used as a measure of the load on the load cell.
The four strain gages on a compression column usually have slightly different sensitivity to strain, and the strains on opposing side faces of a compression column are slightly different because of lack of perfect mechanical symmetry or off-axis loading of the compression column. The accuracy of the output signal from a compression column load cell will thus be impaired when the load cell is subject to off-center or off-axis loads. High accuracy load cells must accordingly be compensated for sensitivity to off-axis loading during calibration of the load cell.
The compensation for loading dependent errors is commonly done by filing of the compression column to restore symmetry. This is a time consuming and messy procedure, which requires skilled craftsmen. Attempts have been made to compensate for loading dependent errors by shunting individual strain gages or half-bridges in the strain gage bridge, but this process is also complicated. Compensation across the longitudinal and the transverse strain gage pairs affect each other, and the shunting resistors for compensation interact with the zero adjustment of the load cell, even if a balanced zero compensation is used.